1. Field of the Invention
The present invention generally relates to an induction motor. More specifically, the present invention relates to an induction motor being supplied in a field winding with a voltage from an alternating current voltage source, while the effective value of the voltage is controlled by means of a phase control circuit or a chopper circuit.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing one example of a conventional induction motor circuit which constitutes the background of the present invention. An alternating current voltage source 1 is coupled through a conduction control circuit 2 and a switch 3 of a field winding 4 of an induction motor. The field winding 4 comprises a main winding 41 and an auxiliary winding 42. A capacitor 5 is connected in series with the auxiliary winding 42. Although not shown in FIG. 1, a rotor is provided in association with the field winding 4, as well known. Thus the FIG. 1 circuit forms a so-called capacitor run motor, in which a phase advancing current is caused to flow by means of the capacitor 5, whereby a starting torque is generated, and the capacitor is maintained as connected even after the start. A conduction control circuit 2 comprises a switching element 21 such as a bidirectional conductive element having a control electrode (triac) and a triggering circuit, not shown, is connected to the gate of the triac 21. A series connection of a resistor 22 and a capacitor 23 is connected in parallel with the triac 21 for the purposes of preventing erroneous ignition or triggering of the triac 21. When a switch 3 is turned on, the field winding 4 is supplied with an alternating current voltage (shown by the solid line curve V in FIG. 2) the conduction phase of which is controlled by the conduction control circuit 2. The magnitude of the field magnetic flux and thus the rotational speed can be continuously changed by changing the phase of a triggering pulse being supplied from the triggering circuit, not shown.
Since an alternating current voltage is suddenly supplied by the conduction control circuit 2 in the conventional induction run motor circuit shown in FIG. 1, a waveform distortion was caused on the occasion of conduction of the triac 21, as shown by the solid line curve V in FIG. 2. Hence, a problem was involved that due to harmonic components included in the distorted waveform vibration is caused in the motor and a noise is generated from the motor. Furthermore, since the field winding 4 has an inductance component, a surge voltage on the occasion of turn-off of the triac 21 becomes high and therefore a protecting circuit becomes necessary, while a phase lag of a current I flowing through the field coil 4 is caused with respect to the applied voltage V, as shown in FIG. 3, with the result that the efficiency and the power factor is decreased. More specifically, the field current I lags by .alpha.=.omega.t: .omega. is the angular frequency of the alternating current voltage source 1 as compared with the voltage V, as shown in FIG. 3, and the above described lag degraded the power factor.
Such a noise as described above would be extremely aggravated when such motor is employed as a prime mover of an electric fan or the like. More specifically, when the effective voltage being applied to the field winding 4 is decreased by means of the conduction control circuit 2 so that the number of revolutions may be decreased, wind singing by a fan is decreased as a matter of course and accordingly a noise due to harmonic distortion becomes more offensive to the ears.
FIG. 4 is a schematic diagram of another conventional induction motor circuit. The FIG. 4 conventional example is disclosed in Japanese Patent Publication No. 18415/1969 published for opposition Aug. 12, 1969 and comprises a tertiary winding or a control winding 6 electromagnetically coupled to the field winding 4, so that a tank circuit may be formed with the control winding 6 and the capacitor 7. A variable resistor 8 is connected in series or parallel with the capacitor 7 in the tank circuit. By adjusting the resistance value of the variable resistor 8, an input current to the tank circuit is controlled, so that a speed control may be made. Since the FIG. 4 conventional example does not employ a conduction control, the same does not cause a noise due to harmonic distortion as encountered in the FIG. 1 conventional example; however, since the peak value itself of the field flux is changed by changing the input current to the tank circuit, another problem is involved that the efficiency of the motor is degraded and a desired starting torque can not be attained according to the circumstances. The FIG. 4 conventional example involves a further problem that since the same does not achieve a speed control by a conduction control no attention has been given to a noise encountered in the FIG. 1 conventional example, which makes it impossible to apply such a circuit as shown in FIG. 4 to such a circuit as shown in FIG. 1. The reason is that the FIG. 4 conventional example achieves a speed control by means of the variable resistor 8 whereas the FIG. 1 conventional example achieves a speed control by means of the conduction control circuit 2.